The present invention generally relates to a braking system for trains and more particularly to a hydraulic braking system for a rail car.
In 1869, the first successful demonstration of a pneumatic or air braking system installed on a train was carried out. Since that time, the railroad industry has focused entirely on pneumatics for braking and stopping trains. Although a considerable amount of evolution has occurred, train braking systems today work under the same principles and practicality as that first air braking system demonstrated in 1869.
In general terms, an air braking system for trains includes a compressor powered by the locomotive which supplies air down the length of the train. When applying the brakes, air enters cylinders which activate pistons. The pistons are pushed out which force brake shoes against the wheel causing the train to slow down. Although presently there are different types of air braking systems, they all work under the same principles and include common mechanisms.
In particular, a typical train air brake system consists of an air compressor located in the locomotive and powered by the engine. The air compressor provides compressed air through a feed valve and a brake valve to a brake pipe. The brake pipe runs the entire length of the train and is connected between rail cars when the cars are coupled together. In communication with the brake pipe, each rail car contains two air reservoirs, a control valve and a brake cylinder. The two air reservoirs include an auxiliary reservoir and an emergency reservoir.
The brake valve to which the conductor of the train has access, typically has four different positions for operating the air braking system. Those positions include the running position, the service position, the lap position and the emergency position.
When the brakes are not being applied, the brake valve is placed in the running position. During this position, air supplied from the compressor is fed to each car via the brake pipe. At each car, the air goes through the control valve and charges the air reservoirs. Air is stored in these reservoirs as potential energy for later applying the brakes as will be described hereinafter.
In order to apply the brakes, the brake valve is first switched to the service position. When switched to this position, the compressor no longer supplies air to the brake pipe. Instead, air is bled off the brake pipe at a controlled rate. This reduction in pressure causes the control valve contained in each rail car to connect the auxiliary reservoirs to the brake cylinders. Air from the auxiliary reservoir then flows into the brake cylinder which activates a piston. The piston in turn moves the brake shoes against the wheel causing the train to decelerate.
After a certain amount of time, and after the brake pipe has lost a particular amount of pressures the brake valve is switched to the lap position during the braking process. In the lap position, the brake pipe is slowly blanked off not allowing any further air to escape. Air pressure within the system then begins to equalize between the brake pipe and the auxiliary reservoir. This equalization causes the control valve contained in each car to reduce flow from the auxiliary reservoir to the brake cylinder which holds the brakes in an applied position. If more braking power is needed, the brake valve is again placed in the service position and the cycle is repeated. To remove the brakes, the brake valve is placed back in the running position.
If an emergency exists and the train needs to be stopped as soon as possible, the brake valve can also be placed in an emergency position. In many trains, an emergency application can be made not only from the brake valve but from any part of the train by a conductor valve. When placed in the emergency position, the brake pipe is valved directly to the atmosphere causing a rapid reduction in air pressure. This sudden drop in pressure not only causes each control valve to connect the auxiliary reservoir to the brake cylinder but also causes the emergency reservoir to be channeled to the brake cylinder. The emergency reservoir and the auxiliary reservoir combine to provide air at a high pressure for applying the brake pads to the wheels with more force. This force typically causes the wheels of the train to lock.
The above-described air brake system for rail cars also contains a built in emergency braking feature. As discussed above, the brakes are applied when air pressure in the brake pipe is decreased. As such, if for any reason the train or cars become uncoupled or any other event occurs in which air is lost in the brake pipe, the brakes automatically apply to the wheels of the cars. This arrangement provides a safe backup system against brake failure which in essence causes the train to stop rather than to lose its brakes.
Also, the train air braking system is typically supplemented with a hand brake system. A hand brake is provided for each car for supplying a braking force for parking the rail cars or stopping the cars when the air source from the locomotive is not available.
The air braking system currently being used by the railroad industry has been proven safe and effective in slowing and stopping rail cars. In fact, the braking system cannot be blamed for increasing derailments or accidents. However, use of the system still has its drawbacks and deficiencies. In particular, the present braking system used in rail cars is very expensive to install and maintain.
For instance, in the August 1992 issue of Progressive Railroading, pages 43-48, in an article entitled "Improving Freight Car Braking," it is estimated that air brake system maintenance costs for freight cars is about 120 Million Dollars annually. Further brake related wheel set costs are estimated at 384 Million Dollars annually while train stop delay costs are about 48 Million Dollars annually. Train stop delay costs include the costs of train stops made due to brake system problems.
Air brake system maintenance costs as estimated above include those costs due to routine testing, maintenance procedures and cycles, and especially the replacement of brake shoes. Because of the increasing weight of freight cars, longer train lengths, and higher speed requirements, current brake shoes wear extremely fast. Also, besides outright wear, brake shoes are commonly replaced due to deterioration, cracking, or taper wear. Taper wear is a condition in which one end of a brake shoe has worn much faster than an opposite end. In having to replace brake shoes, costs are not only incurred for parts and labor but also for downtime of the particular freight car. As such, a need exists for a braking system for a railroad car that does not rely on brake shoes.
Besides brake shoe replacement, wheel repair and replacement is another large maintenance cost item associated with current pneumatic braking systems. In particular, action of the brake shoe against the wheel not only wears down the shoe, but can also have a detrimental effect on the wheel. Specifically, when the brakes are applied the wheel acts as a heat sink. High thermal gradients can form causing thermal cracking in the high stress areas. The cracks can form anywhere on the wheel due to heat dissipation from the brake shoe on the wheel tread. Besides stress cracks, high thermal gradients can also lead to flat spots on the wheel tread. Further, the action of the brake shoe on the wheel can cause spalling. Spalling refers to the formation of martensite on the wheel due to thermal cycles. Martensite is hardened carbon steel which causes wheels to not function adequately, leading to their removal.
Besides exorbitantly high maintenance costs, the presently used pneumatic braking system also includes other disadvantages. For instance, when the brakes are applied in an air brake system, the actuating signal is transmitted down the length of the train by air contained in the brake line. The speed of this signal is therefor limited. Consequently, in longer trains it may take up to five or six seconds before the brake signal is transmitted to the last car. This delay creates a wave action causing the cars to push into each other or "bunch." As such, a need exists for a braking system for a freight car with a faster brake response time.